The present invention relates to surgical cutting instruments, and in particular to cutting instruments configured for use in percutaneous and minimally invasive procedures.
Surgical cutting instruments have been developed that are sized for minimally-invasive or percutaneous access to a site for removal of tissue. One such procedure involves removal of vitreous material from within the eye. Surgical cutting instruments for this type of procedure must have a very small size, because the instrument accesses the interior of the eye directly through the body of the eye. The cutting instruments integrate aspiration with the cutting function to withdraw the tissue as it is excised. In order to accommodate this aspiration function, these cutting instruments are typically of the tube-within-a-tube type in which an inner tubular cutter moves within a larger outer cannula. Vacuum is drawn through the inner tubular cutter to pull tissue severed by the cutter back through the instrument.
Tube-within-a-tube cutting instruments typically incorporate either rotary or reciprocating inner tubular cutters. The rotary cutter includes a cutting edge that rotates past an opening in the outer cannula through which tissue is drawn and severed. In the reciprocating cutter, the inner cutter translates back and forth within the outer cannula. The end of the reciprocating cutter defines a cutting edge that severs tissue extending through the opening in the outer cannula, usually on the forward stroke of the cutter.
In many cases, prior tube-within-a-tube cutters, especially reciprocating cutting instruments, experienced difficulties in cleanly severing target tissue drawn into the opening of the outer cannula. Such difficulties have been manifested in either a failure to actually sever the target tissue, or incompletely cutting the tissue in a given stroke of the inner cutter. In either case, these difficulties significantly compromise the cutting efficiency of the instrument. Difficulties of this type can pose a particularly troublesome problem for certain tissue, such as the vitreous tissue of the eye. A failure to fully sever the target vitreous tissue can lead to “stringers”—vitreous tissue that becomes lodged in the cutting instrument as it is removed from the eye.
In order to address this problem, a “hinged blade” inner cutter was pioneered, as disclosed in U.S. Pat. No. 5,782,849, to Michael E. Miller. General features of this cutting instrument are illustrated in FIGS. 1-5. In particular, the cutting instrument 10 includes a handpiece 12 that supports an outer cannula 20. A tubular inner cutter 22 is disposed for reciprocation within the outer cannula. The cutting head 26 of the inner cutter defines a cutting edge 24 (FIG. 2) that traverses the tissue opening 21 in the outer cannula to sever tissue T. An aspiration vacuum A (FIG. 3) draws a vacuum within the tubular inner cutter through aspiration tube 14. The reciprocating unit reciprocates the inner cutter 22 in the direction R (FIG. 2) by a drive mount 16 connected to a suitable source of linear motion.
As shown in FIG. 3, the outer cannula has an inner diameter D1 greater than the outer diameter D2 of the inner cutter. This difference in diameters provides a running clearance for low friction reciprocation of the inner cutter within the outer cutter. In prior reciprocating cutting instruments, this running clearance was maintained through the entire stroke of the inner cutter, including at the tissue opening 21.
In accordance with one aspect of the invention disclosed in the '849 Patent, a hinge slot 23 is formed in the inner cutter so that the cutting head 26 can pivot in the direction P (FIG. 3) as the cutting edge 24 contacts the tissue T. In other words, as the cutter advances into the tissue T, the tissue tends to resist the forward movement of the cutter. This resistance thus causes the cutting head to pivot in the direction P. As the cutting head 26 pivots upward, it forms an essentially zero clearance between the cutting edge 24 and the edge of the tissue opening in the outer cannula. This zero clearance allows the inner cutter 22 to cleanly sever the tissue segment T1 that has been drawn into the outer cannula. On each stroke of the inner cutter 22, new tissue segments T1 are severed while the vacuum draws previously severed tissue T2 back through the cutting instrument.
Another embodiment of an inner cutter is illustrated in FIG. 4. In this embodiment the hinge slot extends along most of the length of the inner cutter 22′ to define a body portion. The cutting head of this embodiment pivots in response to the resistance offered by the tissue, and provide an alternative method of securing the inner cutter to the reciprocating unit.
In a modification of the inner cutter 22, the cutting head 26 may be “pre-bent” at an angle α relative to the length of the tubular inner cutter, as shown in FIG. 5. A user then inserts this “pre-bent” inner cutter into the outer cannula when assembling the cutting instrument 10. It has been found that this “pre-bend” characteristic can optimize the cutting performance of the instrument 10 when operating on difficult tissues. For instance, the vitreous tissue of the eye has viscous properties causing it to offer insufficient resistance to the cutting head 26 as the reciprocating unit advances the inner cutter. In the absence of sufficient resistance from the tissue, the cutting head may not pivot fully upward to form the desired zero clearance. In order to ensure a clean and complete cut, the cutting head 26 is bent upward slightly, as shown in FIG. 5, so that when the cutting edge 24 contacts the tissue at the tissue opening 21, it does so at a nearly zero clearance even in the absence of sufficient resistance from the tissue to cause the cutting head to pivot. A modified inner cutter 22″ is shown in FIG. 6 in which the hinge slot 23′ is enlarged to optimize the pivot action of the pre-bent cutting head 26.
The pre-bent cutting head shown in FIGS. 2-6 allows the reciprocating cutting instrument to efficiently sever and aspirate a wide range of tissues, including the troublesome vitreous tissue. Moreover, commercial instruments incorporating the hinged-blade and pre-bend features are capable of cutting speeds that greatly exceed the capabilities of prior tube-within-a-tube cutting instruments. However, the pre-bent cutting head shown in FIGS. 5-6 inherently reduces the running clearance between the outer cannula and the inner reciprocating tubular cutter, making contact between the two components more likely. This contact produces sliding friction that can increase the load upon the drive mechanism used to reciprocate the inner cutter. This increased load manifests itself in a reduction in cutting speed or in the need for a larger drive motor. In addition, the sliding friction can generate heat along the length of the instrument, which may not be desirable in certain surgical applications. Therefore, further improvement of this successful cutting instrument is always desirable.